Fireplace combustion air duct



FIG. 1 is a side elevational view of a fireplace combustion air duct, showing my new design;

FIG. 2 is a front elevational view thereof.

FIG. 3 is a top plan view thereof.

FIG. 4 is a bottom plan view thereof. 

The ornamental design for a fireplace combustion air duct, as shown. 